Semiconductors have enabled nearly all electronics (e.g. transistors, diodes, etc), communication systems, optoelectronics (e.g. light emitting diodes, laser diodes, photodetectors), photovoltaics, sensors, and other such applications. This is due to their unique place between insulators and conductors, and the corresponding ability to switch, produce gain, and emit light.
The electronic properties of semiconductors arise from their crystalline lattice, with the regular order giving rise distinct energy bands of allowed energy states separated by forbidden gaps. The nature of this forbidden gap, or band gap, is the key parameter which dictates what applications a particular semiconductor is suited for; it controls whether the semiconductor emits light (direct vs. indirect transition) and at what wavelength, as well as the charge carrier concentration and carrier mobility. FIG. 1 provides an illustrative plot of intrinsic carrier concentration versus band gap energy. FIG. 2 provides another illustrative plot of wavelength versus band gap energy.
Modifying the atomic spacing/composition of the semiconductors and/or externally applying strain to the semiconductors may allow tuning of fundamental physical and electronic properties of semiconductors (e.g. band gap, carrier mobility, density of states, emission wavelength, optical absorption, etc.). For example, externally applying strain may improve semiconductor device performance by enhancing the electron mobility for transistors, modifying states for lasers, enabling faster transistor switching speeds and tuning photon absorption properties. FIGS. 3A and 3B provide schematics of conventional two dimensional planar semiconductor devices comprising an unstrained direct band gap semiconductor 302 and a strained direct band gap semiconductor 304, respectively.
Conventional techniques for applying strain to semiconductors typically rely on interfacial lattice mismatch, thus limiting the applied strain to the interfacial or near interfacial regions. Moreover, utilization of these conventional techniques strongly couples the applied strain to the choice of semiconductor materials. For instance, to achieve a desired strain a specific combination of semiconductors must be used, which will ultimately dictate the fundamental properties of the resulting device. A large share of current semiconductor technology involves the use of the following semiconductor material systems: Si/SiGe, SiC, GaAs/AlGaAs, GaN/AlGaN, InP/InGaAsP etc. However, manufacture of these heterostructures requires advanced high vacuum deposition methods (e.g. metallorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE)), which contribute greatly to the overall cost in device manufacturing.
Accordingly, conventional strained semiconductor technology is limited in the materials that can be used and the volume of material that can be strained.